The present invention relates generally to implantable medical devices for treating cardiac dysrhythmias, and more particularly to a device which is both upgradable by programming to treat dysrhythmias according to a patient's changing needs over time, and responsive to cessation of a shock waveform delivered by the device to the patient's heart to apply post-shock pacing pulses to the affected chamber for a period of time sufficient for the heart to recover to normal heart rate.
Selection of an appropriate pacemaker to be implanted, for example, is made after patient evaluation, diagnosis of the disorder (e.g., dysrhythmia), and determination that artificial pacing can be an effective therapy to alleviate the dysrhythmia. As with any therapy, consideration of side effects and contraindications is vital. Techniques of selecting the respective proper pacing modes for treating particular dysrhythmias are set out in algorithmic form; for example, in M. Schaldach, Electrotherapy of the heart, Springer-Verlag, Berlin (1992).
The '559 patent and the '560 application describe multi-mode (pacing, cardioverting, defibrillating) devices implemented with a full range of features which are not all required to be operational at the time of implantation, and which may be upgraded or modified non-invasively while surgically implanted, whenever the progress of the patient's underlying disease or deficiency dictates, through remote programming. For many patients, progression of cardiac disease or disorder has necessitated surgical removal (explanation) of a now less-effective or ineffective device, and implantation of a new device capable of effective treatment of the current dysrhythmia.
U.S. Pat. No. 5,609,613 ("the '613 patent"), commonly assigned herewith, discloses improvements in artificial pacing for various conditions of patient rest, exercise/activity, and atrial dysrhythmia, with enablement of automatic mode switching between dual-chamber and single-chamber modes. The advent of a fully automatic DDD pacemaker allowed adapting the ventricular pacing rate to depend on rate of the sensed intrinsic atrial signal, and AV synchrony. But about 50% of the DDD pacemaker implant patients were found to experience atrial sensing problems or atrial instability, with underlying atrial rhythm disorders, and these patients had to be switched from DDD pacing mode to the simpler VVI mode by pacemaker replacement. Subsequent improvements in sense amplifiers, electrodes for atrial leads, and timing cycles (e.g., dynamic AV delay and refractory periods) enabled DDD pacers to provide effective therapy to some of this patient population, but a considerable percentage of patients with dual chamber implants were still found to have inadequate atrial rates. Some one-third of patients with sick sinus syndrome (characterized by sinoatrial (SA) arrest or SA exit block) exhibit overly high atrial rates and accompanying atrial fibrillation, atrial flutter, or sinus tachycardias including atrial reentry tachycardias and ectopic tachycardiac events (which develop from a focus other than the SA node), as well as slow heart rates.
Rate-adaptive pacing techniques are used to monitor artificial pacing rate and intrinsic heart rate, and for controlling the pacing rate to meet the patient's metabolic needs. For example, the RELAY (a trademark of Sulzer Intermedics Inc.) dual-chamber, multi-programmable, accelerometer-based rate-adaptive cardiac pulse generator not only varies the pacing rate according to the patient's level of activity (or lack of activity, i.e., resting) and body position but also monitors adequacy of triggered pacing of the atrium. In the RELAY.TM. pacing system, the maximum programmable rate (MPR) is supplemented by a slower interim rate which is greater than the lower rate limit of the device--a ventricular tracking limit (VTL). The pacing rate moves from its base rate to the VTL, conditioned on a high atrial sensed rate without patient exercise. This conditional VTL (CVTL) may be programmed "on" (i.e., as an operating feature of the device) to undergo a controlled jump to the interim rate when a high atrial intrinsic rate is sensed but the accelerometer fails to confirm patient exercise. CVTL is overridden when MPR is programmed "on" and the rate calculated from patient exercise exceeds the programmed base pacing rate by a preselected amount--e.g., 20 beats per minute (bpm). At this accelerometer-based rate threshold, the pacemaker pulse generator restores 1:1 AV synchrony up to the MPR.
The '613 patent discloses a rate-adaptive, dual chamber pacemaker in which the VTL is a dynamic rate, and the ventricular pacing rate is controlled through several different rate zones, based on a combination of (i) dynamic adjustment of VTL according to the accelerometer-based activity signal, and (ii) automatic mode switching from a dual-chamber to a single-chamber mode with reversion to the dual-chamber mode based on an atrial cut-off rate and a programmable rate criterion. Among other things, a mode switch rate (MSR) is designated--above the MPR--that represents an atrial rate unlikely to be exceeded by even a healthy person with a normal cardiovascular system. Mode switching from DDDR to VVIR (the R suffix indicating rate-adaptive functions), for example, may be set to take place automatically when sensed atrial rate exceeds MSR for a programmed number of consecutive cardiac cycles, with reversion to dual-chamber operation occurring automatically when sensed atrial rate falls below MSR for one cycle.
While techniques such as automatic mode switching are highly desirable to avoid a need for physician reprogramming, instances arise in which progression of cardiac disease mandates a more permanent change in device functionality, or additional features not previously required for control of the patient's dysrhythmia. In the '559 patent, an implantable defibrillator is designed to be upgradable non-invasively by selectively programming and re-programming the device in a secure manner each time the patient's condition undergoes a significant change, to provide the minimum functional capabilities required to treat the patient's current dysrhythmia. Various therapeutic features and capabilities of the implanted device which are not required for treating the patient's current disorder may be rendered inactive or disabled for the time being and selectively made available from time to time thereafter when and as prescribed for treatment of an advanced stage of progressive cardiovascular disease, without a requirement of surgical removal and replacement of the implanted device. Initial cost to the patient is relatively low, if applied only to the limited features of the device which have been activated. As additional features are activated in the course of treatment of an advancing disease, additional charges may be imposed to allow recovery of costs of development, manufacture, distribution and marketing associated with those features.
The '560 application discloses an implantable, programmable multi-mode cardiac pacemaker electrical function generator which is adapted to be upgraded selectively and non-invasively, through programming, in a similar manner.
All such programming of functions requires imposition of appropriate security measures, as well as additional charges on the patient's account to compensate the device manufacturer for the extended function(s). To avoid the possibility of unauthorized upgrade, a security code or key supplied by the manufacturer is required to allow the desired mode restoration from a dormant state. Any of several distinct and different security codes which are unique to the particular implanted device may be used for this purpose, or each the security codes may be associated with a respective distinct and different mix of enabled and disabled pacing or functional operating modes, to obtain a particular mix only with a specific one of the codes.
An initially simple and inexpensive device is converted by these means to a more sophisticated device without subjecting the patient to additional surgery. The typical cardiac patient does easily tolerate the physical, mental, emotional, and economic toll of multiple operations which may range from an initial relatively simple implant device to successively more complex devices to meet the advancing needs for therapy dictated by progressive heart disease. Added to this is the care required to be delivered to the patient by physician, surgical, and hospital services, and limited care mandated by government-imposed cost containment especially in the case of patients of advanced years who generally are candidates for such implant devices.
The present invention employs not only the capability to be upgraded by secure programming, but is also directed toward a device having the capability to provide improved diagnostics and therapies to treat tachyarrhythmias. In the majority of cases, tachyarrhythmias are associated with reduced myocardial contractility, where underlying structural disease of the heart is often responsible as well for the occurrence of threatening tachyarrhythmic events. Under normal and usual conditions, treatment with appropriate medication such as ACE inhibitors, digitalis and diuretics can maintain a sufficient cardiac output. But where arrhythmia is present, and especially on occasions following treatment of an arrhythmia, reduced myocardial performance is observed, with severely compromised cardiac output.
Studies conducted by the applicant have shown that VVI pacing with a lack of synchronization of atrial and ventricular activation has a serious adverse effect on myocardial performance in these patients. Normally, an atrial contraction is followed by ventricular activation, which are observed as a P-wave and QRS-complex, respectively, in the ECG, but if the patient is subjected to a defibrillation shock from an implanted device the post-shock rhythms can also severely compromise myocardial performance. The undesirable rhythms result from an alteration of the sinus node induced by the shock(s), and even though the node function may remain adequate with respect to its basic rate, the high energy delivered by the shock has an adverse effect on its chronotropic function. Consequently, ventricular defibrillators tend to reduce the effectiveness of back-up VVI pacing.
These same studies by the applicant have demonstrated that two mechanisms in particular appear to cause reduced cardiac output following delivery of a defibrillation shock. First, the high energy shock itself impacts adversely on the myocardial function, which may be attributable to a type of electroporation that alters the basic contractility. Second, the high energy shock deleteriously affects the chronotropic recovery and the automaticity of the sinus node, which results in considerable lengthening of the phase for depolarization of the atrial natural pacemaker cells. In practice, the atrial rate may fall to values of 30 to 40 beats per minute (bpm), which is much too low in light of the severely compromised myocardial function.
Additionally, in many patients with normal sinus node, a normal conduction in the atrio-ventricular (AV) node is present in antegrade direction, and hence, retrograde conduction from the ventricle to the atrium can occur. Under conditions of VVI pacing, "pacemaker syndrome" is present in which a retrograde activation of the atrium results in cannon waves caused by contraction of the right atrium at a time when the tricuspid valve is closed by contraction of the right ventricle. Consequently, a pumping function of the atrium will lead to a shift of blood in a direction opposite from normal, into the pulmonary veins, rather than toward the right and left ventricle. Conversely, when the mitral valve opens, the atrium is empty which leads to a diminished filling of the ventricle and a resulting serious deficiency in ventricular performance. In practice, these patients exhibit a blood pressure of 60 to 70 millimeters of mercury (mm Hg) during VVI pacing, compared to regular blood pressure of 100 to 110 under conditions of an AV synchronized rhythm such as sinus rhythm or a sequential pacing rhythm following a pacing of the atrium and the ventricle.
It is therefore a principal aim of the present invention to provide an implantable medical interventional device and device-implemented method to improve cardiac output following high energy shocking of the heart by post-shock pacing of the heart for a period sufficient to restore normal sinus rate.
A related aim of the invention is to provide an implantable medical interventional device and device-implemented method which enables temporary post-shock pacing of the atrium and ventricle in treatment of tachyarrhythmias, and wherein the device may be functionally upgraded non-invasively by secure programming.